1. A review of the Evolution of the Model of the Atom Chapter 13-1
Models of the Atom
A review of the Evolution of the Model of the Atom
Chapter 13-1

2. The Different Models of the Atom
In the XIX, Thomson described the atom as a ball of positive charge with a number of electrons.
Said the atom was
like plum pudding.
Thomson Model

3. Rutherford Model
In the Early XX, Rutherford showed that the most of the atom’s mass is concentrated in a small, positively charged nucleus.
Atom is mostly empty space.

4. Bohr Model
After Rutherford, Bohr proposed that electrons travel in definite orbits around the nucleus.

5. Quantum Mechanical Model
Modern atomic theory describes the electronic structure of the atom as the probability of finding electrons within certain regions in space.

6. Energy Levels/Shells Energy Level Quantum
The energy level of an electron is the region around the nucleus where the electron is likely to be moving.
It is the amount of energy required to move an electron from its present energy level to the next higher one.

7. Electron Configuration
What is it?
The configuration is described by 4 quantum numbers.
Just like a point can be described by the (x,y) coordinates
An electron can be described by 4 quantum numbers.

8. Electron Configuration
In the atom, electrons and the nucleus interact to make the most stable arrangement possible.
The way in which electrons arranged around the nuclei are called electron configurations.

9. Principal Quantum numbers (n)
Also known as Principal energy level.
Each Principal quantum number refers to a energy level in an atom.
Look at the periodic table.
Along the left side you will see numbers from 1 to 7.
These represent the energy levels of electrons for that row of atoms. n=1, n=2 , n=3 …

10. 2s (1orbital) 2p (3 orbitals)
Principal energy Level
Number of sublevels
Type of sublevel
n = 1 1
1s (1 orbital)
n = 2 2
2s (1orbital) 2p (3 orbitals)
n = 3 3
3s (1 orbital) 3p (3orbitals)
3d (5 orbitals)
n = 4 4
4s (1 orbital) 4p (3 orbitals)
4d (5 orbitals) 4f (7 orbitals)

11. Electron Arrangement in Atoms
The Three Rules that tell you how to find the electron configuration of atoms
Aufbau Principle
Pauli exclusion Principle
Hund’s rule

12. Electrons enter orbitals of lowest energy first.
Aufbau Principle
Pauli Principle
Electrons enter orbitals of lowest energy first.
An atomic orbital may describe at most two electrons
To occupy the same orbital, the two electrons must have opposite spins

13. Hund’s Rule
When electrons occupy orbitals of equal energy, one electron enters each orbital until all the orbitals contain one electron. Only after all the orbitals are filled will electrons pair-up.
This applies to the p, d and f sublevels.

14. Electrons enter orbitals of lowest energy first.
And so on

15. Notation for Quantum Numbers
Principle Quantum Number (n)
1s2
Number of Electrons
Angular Momentum

16. 1s2 In other words This would the second electron
The first energy level
the s sublevel of the 1st energy level

17. An atomic orbital may at most describe two electrons.
See pg 365 figs 13-4 and 13-5
s orbital can hold 2 electrons – it has 1 orbital
p orbitals can hold 6 electrons – each of the 3 orbitals can hold 2 electrons
d orbitals can hold 10 electrons – each of the 5 orbitals can hold 2 electrons
f orbitals can hold 14 electrons – each of the 7 orbitals can hold 2 electrons

18. Only after all the orbitals are filled will electrons pair-up.
An electron enters each orbital one at a time until all the orbitals contain one electron.
Only after all the orbitals are filled will electrons pair-up.
For example:
Carbon has 6 electron so it’s configuration is: 1s2 2s2 p2
n=1
and it fills in this order
n=2

19. THE ULTIMATE TOOL

20. Sample Test Question
Write the complete electron configuration for each atom.
Argon
Carbon

21. Answers
1S2 2S2 2P S2 3P6
1S S2 2P2
Argon
Carbon